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If I have two independent (unentangled) qubits, let's say one in state |1> and the other one in some superposition state with equal amplitudes and arbitrary phases. If I measure the qubit that is in the superposition state (and let's say get 0), it seems that the remaining phase is kicked back to the first (non-measured) qubit. So the state of the first (non-measured) qubit changes as a result of the measurement of the second qubit. But the qubits are independent and are not supposed to know about each other and impact each other. So why is the phase transferred to the first qubit?

enter image description here

here is the qiskit code to illustrate the question:

# some arbitrary state:
theta = [np.pi/3,np.pi/4]
a = 1/np.sqrt(2)*np.array([np.exp(1j*theta[0]),np.exp(1j*theta[1])])

qc = QuantumCircuit(2,1)
qc.initialize(a,1)  # set q[1] to a
qc.measure(1,0)
execute(qc,svsim).result().get_statevector()  # get the post-measurement state
EugeneB
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    While this isn't off-topic here, you would likely have better luck if you ask at the Stack Exchange community [quantumcomputing.se]. – John Coleman Nov 09 '22 at 12:01

1 Answers1

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This is due to the difference between global and relative phase. What you can see here is a difference in global phase only which is not observable. The difference between global and relative phase is well explained here: https://quantumcomputing.stackexchange.com/questions/5125/what-is-the-difference-between-a-relative-phase-and-a-global-phase-in-particula

In your case the result is the expected one as the state you get is indistinguishable from the state [0, 1].

user2490698
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